The Tyranids economy starts off pretty slow but it can gather speed very quickly. You need to keep them off points if possible and destroy those Reclamation Pools because they increase the Nids economy as well. The IG economy is no slouch though, it's probably the best of the other races, tech to vehicles as soon as you can and Hellhounds should make your life much easier.
Flamers are good but don't rely too much on them. Genestealers are powerful but are quite fragile so focus firing on them and they should go down quite easily. The Broodlord is a different kettle of fish. Stay away at all costs. Grenade Launchers when available should help enormously due to the area of effect and knockback. Aggressive use of bunker buildings should help a lot too. Build Infantry Commands quite close to the enemy so you can fall back to it with your troops and use its guns to soften the enemy up, it's quite effective because of its cheapness. Also you can keep a Enginseer nearby and use it to repair and help out with the guns if necessary.
IG Turrets are great but don't spend all your time building them, build a few in key areas, and support them with your troops and an Enginseer to repair. Turtling with IG works better than with other races but being attack minded works better generally.